Geographia Polonica 2017, Volume 90, Issue 1, pp. 39-51 https://doi.org/10.7163/GPol.0077 INSTITUTE OF GEOGRAPHY AND SPATIAL ORGANIZATION POLISH ACADEMY OF SCIENCES www.igipz.pan.pl www.geographiapolonica.pl FACTORS INHIBITING CONVECTION UNDER CONDITIONS OF EXTREME ATMOSPHERIC INSTABILITY Angelika Palarz • Daniel Celiński-Mysław Department of Climatology Jagiellonian University Gronostajowa 7, 30-387 Kraków: Poland e-mails: [email protected] • [email protected] Abstract The paper identifies mechanisms that potentially inhibit convection at a time when extreme values of selected atmospheric instability indices are recorded. The study involved six indices (LI, SI, CAPE, KI, SWEAT, TTI). Data sources involved records from three Polish data stations collecting upper air soundings and covered the pe- riod 2005-2014. Additional data were obtained from SYNOP codes on present and past weather and reports on severe meteorological phenomena from the European Severe Weather Database. The methodology adopted allowed the selection of 26 cases where no convective phenomena were observed despite extreme atmospheric instability. A detailed analysis demonstrated that the occurrence of isothermal or inversion layers in the lower and middle troposphere were the most frequent mechanisms inhibiting the vertical air movement. Convection was also inhibited when the area was free from the influence of atmospheric fronts, convergence zones, low- pressure troughs or when high altitudes of LCL occurred. Key words convection inhibition • atmospheric instability indices • Poland • upper air sounding Introduction (DeRubertis 2006; Venkat Ratnam et al. 2013), or climatological reanalyses (Brooks et al. Atmospheric instability indices are popular 2007; Riemann-Campe et al. 2009). These in forecasting powerful convective phenome- studies primarily involved the indices most na. Their verifiability, however, can be strongly frequently used in synoptic practices, i.e. Con- affected by the occurrence of a range of fac- vective Available Potential Energy (CAPE) and tors inhibiting the development of the vertical Convective Inhibition (CIN) (Blanchard 1998; movement of air. Published research on the Romero et al. 2007; Brooks 2009; Riemann- temporal and spatial variability of instability Campe et al. 2009). The other indices, includ- indices used either upper air sounding data ing the Lifted Index (LI), Severe Weather Threat 40 Angelika Palarz • Daniel Celiński-Mysław Index (SWEAT), Showalter Index (SI), K-Index inhibition of the development of convection (KI) and Total Totals Index (TTI) featured in far under conditions of extreme atmospheric fewer studies (Siedlecki 2009; Venkat Ratnam instability. et al. 2013). The relationship between the values Materials and methods of atmospheric instability indices and the occurrence of convective phenomena has The study employed upper air sounding been studied with regards to thunderstorms data from the database of the University (Sanchez et al. 2009; Gubenko & Rubinshtein of Wyoming’s Department of Atmospheric 2015), hailstorms (Palencia et al. 2010; Hand Science (http://weather.uwyo.edu/). This & Cappelutti 2011) and tornadoes (Brooks included the values of the six indices most et al. 2003; Romero et al. 2007). These studies frequently applied in convection forecasting: identified a number of threshold values (with CAPE (Convective Available Potential En- wide spatial variation), the crossing of which ergy), LI (Lifted Index), SI (Showalter Index), could lead to severe weather events. They also KI (K Index), SWEAT (Severe Weather Threat demonstrated that powerful convective phe- Index), TTI (Total Totals Index). Data from nomena could develop in relatively low insta- three Polish stations (Legionowo, Łeba and bility environments. Such occurrences involve Wrocław), as taken at 12:00 UTC during the high kinematic parameters (e.g. strong wind period 2005-2014 were used in this paper. shears) which are mostly observed during the Extreme values of the atmospheric instability cool half of the year (Bentley & Mote 2000; indices were defined using their probability Burke & Schultz 2004; Van den Broeke et al. of occurrence: below the 5th percentile for 2005; Sherburn & Parker 2014). LI and SI and above the 95th percentile for Major factors known to help the devel- CAPE, KI, SWEAT and TTI. The analysis was opment of convection include unstable at- then restricted to just those cases in which mosphere, a low lifting condensation level, at least three of the six indices met the cut- a large concentration of water vapour in the off conditions. lower troposphere and large scale mecha- CAPE is an amount of energy that is avail- nisms assisting in the development of con- able during convection, and is calculated by in- vection (e.g. fronts, convergence lines and tegrating vertically the local buoyancy of the low-pressure troughs). Also kinematic and parcel from the level of free convection (LFC) thermodynamic conditions play a significant to equilibrium level (EL). The formal definition role. There is a certain body of research avail- is given by (Moncrieff & Miller 1976): able in this area, including studies on the im- EL pact of Convective Inhibition (CIN) and of the § TTvp ve · Lifting Condensation Level (LCL) on the oc- CAPE ³ J㨠¸ G]ã>J/] kg LFC T currence of powerful convective events which © ve ¹ were presented by Chaboureau et al. (2004) where: and Davies (2004). A number of authors, g – is the acceleration due to gravity, including Chaboureau et al. (2004), Wong Tvp – is the virtual temperature of the parcel, and Dessler (2005) and Riemann-Campe Tve – is the virtual temperature of the environment. et al. (2009), to name but a few, wrote about the inhibitive role of isothermal and inver- To the investigation of the weather condi- sion layers in the development of upward air tions in selected cases, the energy available movement. to a parcel of air originating at the surface This study builds on the previous research and being lifted to its level of free convection on the inhibition of vertical air movement (SBCAPE – Surface-Based Convective Availa- in the lower troposphere. Its overall objec- ble Potential Energy) and the energy of strong tive is to identify mechanisms leading to the downdrafts (DCAPE – Downdraft) were used. Geographia Polonica 2017, 90, 1, pp. 39-51 Factors inhibiting convection under conditions of extreme atmospheric instability 41 LI is the temperature difference between V850 – is the 850 hPa wind speed (in knots), the environment temperature at 500 hPa V500 – is the 500 hPa wind speed (in knots), (T ) and the temperature of a parcel lifted e500 the formula: adiabatically to 500 hPa (T ), given p Surface " 500 (dd – dd ) – describes the directional veering by (Galway 1956): 500 850 of wind with high. LI = T – T [°C] e500 p Surface " 500 CAPE, LI and SI are based on Lifted Parcel Lifting parcel from the surface is done Theory, which represents the adiabatic lift- in order to capture low level boundary layer ing of a parcel in an environment described temperature and moisture conditions while by radiosounding, while TTI, KI and SWEAT are reducing diurnal effects. calculated by combined measurements of the SI is defined as the difference between the thermal and moisture properties, as well as the observed temperature at 500 hPa (Te500) and wind shear in the low and mid-troposphere. the temperature of an air parcel after it has In order to identify days when no convec- been lifted adiabatically to 500 hPa from tive events were observed (such as thunder- 850 hPa (Tp850 " 500) (Showalter 1953): storms, heavy precipitation or strong wind) despite the extreme values of instability indi- SI = T – T [°C] e500 p 850 " 500 ces, SYNOP reports from 6:00, 9:00, 12:00, In the case of SI an air parcel is lifted from 15:00 and 18:00 UTC (http://www.ogimet. 850 hPa where localized low level influences com/) were used. The weather data was are greatly reduced. limited to within 150 km of each upper air Total Totals Index is expressed as the tem- sensing station, i.e. 14 stations around Łeba, perature difference between 850 (T850) and 15 around Legionowo and 16 around Wrocław 500 hPa (T500) and the difference between dew (Fig. 1). The results were then verified using the point at 850 hPa level (Td850) and the tempera- European Severe Weather Database reports ture at 500 hPa (Miller 1972): (http://www.eswd.eu/) and, in selected cases, also data from the lightning location system TTI = (T – T ) + (T – T ) [°C] 850 500 d850 500 (http://www.lightningmaps.org, http://www. K-Index is s similar to Total Totals Index. Ad- pogodynka.pl, http://www.wetterzentrale.de). ditionally this index takes into account moist The study identified the following factors air at 700 hPa contributing to air mass thun- that may affect the development of vertical derstorms development. KI increases with air movement: large scale mechanisms assist- decreasing static stability between 850 and ing in the development of convection (includ- 500 hPa, increasing moisture at 850 hPa, and ing fronts, convergence lines and low-pressure increasing relative humidity at 700 hPa. The troughs), the occurrence of inhibition layers K index is defined as follows (George 1960): (isothermal and inversion), the presence of wa- ter vapour in ground-level layers, the lifting con- KI = (T – T ) + T – (T – T ) [°C] 850 500 d850 700 d700 densation level occurring at low altitude (the Severe Weather Threat Index incorporates lower the